The invention relates to an apparatus for the measurement of fluid flow. More particularly, the invention relates to a mass airflow meter wherein the electrical properties of a temperature dependent element vary in proportion to the mass of air flowing across the element.
Mass airflow meters containing temperature dependent elements are known for providing an electrical measurement of the mass of air inducted into an internal combustion engine. In one approach, a temperature dependent element such as a resistive wire is electrically heated by passing electrical current therethrough to maintain a substantially constant temperature. Since the current drawn by the wire varies with the mass of cooling air flowing across the wire, a measurement of the current provides a measurement of the airflow. A problem with the hot wire approach is that airborne particulate striking the wire will eventually change its resistance, and therefore its measuring accuracy, due to such factors as erosion and accumulation of particulate residue.
Various solutions to the problems caused by airborne particulate have been tried. For example, U.S. Pat. No. 4,412,449, to Eirmann, discloses placing a protective screen upstream of a temperature dependent element to shield the element from the particulate. In another approach, U.S. Pat. No. 4,213,335, to Peter et al, discloses a temperature dependent element having a geometry wherein the surfaces opposing the airflow are small in area when compared to surfaces lying substantially in the direction of the airflow. It is alleged that less particulate will strike the opposing surface than the parallel surfaces. Still another approach is disclosed in U.S. Pat. No. 4,457,169, to Lauterbach et al, wherein a temperature dependent resistor is disposed in a gap extending parallel to the airflow, and a deflecting body bridging that gap is disposed upstream of the gap. The spacing between the deflecting body and the gap defines two inlet conduits which discharge into the gap at an acute angle. The inertia of the airborne particulate impedes the particulate from turning into the gap.
A disadvantage with all of the above approaches is that protection against airborne particulate is provided in only one direction of airflow, protection is not provided during conditions of either backflow or backfire. When a backflow occurs, such as during low speed and high load operation, air will flow backwards across the temperature dependent element without first passing across a screen or a deflecting body. The temperature dependent element will therefore be exposed to airborne particulate. In addition, the backflow will be erroneously measured as intake air unless additional apparatus is employed to correct for the measurement of backflow. Similarly, when an engine backfire occurs, the temperature dependent element will be exposed to both airborne particulate and soot from the engine.
An approach to deal with the backflow problem is disclosed in both U.S. Pat. No. 4,420,972, to Kuroiwa et al and U.S. Pat. No. 4,494,405, to Oosuga et al, which show a bypass air passageway communicating with the main inducted air passageway. The geometry of the passageway is configured to separate particulate from the temperature dependent element. A disadvantage of this approach, however, is that the bypass geometry will also cause air turbulance resulting in a poor signal-to-noise ratio for the mass airflow meter.
There still remain the above problems of reducing particulate deposition on a temperature dependent element, isolating the element from both backflow and backfire, and providing an airflow across the element with minimal fluctuations in velocity or turbulence. None of the uncovered prior approaches provides a solution to all of these problems. It would be advantageous to develop an airflow meter which is sufficiently accurate in a relatively dirty environment to be used as part of an engine control apparatus for an automobile. These are some of the problems this invention overcomes.